1. Field of the Invention
The present invention relates to a hydraulic control apparatus for an automatic transmission of an automotive vehicle.
2. Description of the Prior Art
In Japanese Patent Laid-open Publication No. 63-210443, there has been proposed a hydraulic control apparatus for an automatic transmission of the type which has a change-speed gear train composed of first and second hydraulic clutches and first and second hydraulic brakes. The hydraulic control apparatus includes first, second, third and fourth solenoid valves respectively for controlling fluid under pressure supplied to and discharged from the first hydraulic clutch, for controlling fluid under pressure supplied to and discharged from the second hydraulic clutch, for controlling fluid under pressure supplied to and discharged from the first hydraulic brake, and for controlling fluid under pressure supplied to and discharged from the second hydraulic brake. In the automatic transmission, the change-speed gear train establishes a first drive power train when the first hydraulic clutch and brake are activated under control of the first and third solenoid valves, a second drive power train when the first hydraulic clutch and second hydraulic brake are activated under control the first and fourth solenoid valves, a third drive power train when the first and second hydraulic clutches are activated under the first and second solenoid valves, and a fourth drive power train when the second hydraulic clutch and brake are activated under control of the second and fourth solenoid valves.
In such a conventional hydraulic control apparatus for an automatic transmission, all the solenoid valves each are in the form of a normally open valve which is supplied with fluid under pressure at its output fluid passage in a deactivated condition, and there are provided a first spool valve for fail-safe for disengaging the first hydraulic brake in a condition where at least one of the first, second and fourth solenoid valves is supplied with fluid under pressure at its output fluid passage and a second spool valve for fail-safe for disengaging the second hydraulic brake when at least one of the second, third and fourth solenoid vales is supplied with fluid under pressure at its output fluid passage. In operation, if all the solenoid valves are deactivated due to disconnection of an electric power supply system, the first hydraulic brake is disengaged under control of the first spool valve, the second hydraulic brake is disengaged under control of the second spool valve, and only the first and second hydraulic clutches are engaged to establish the third drive power train. If in such an instance, at least one of the first and second spool valves fails to disengage either one of the first and second hydraulic brakes, the third drive power train may not be established.